Conventional devices used for optical coupling between waveguides, particularly between an integrated waveguide and an optical fiber, have dealt primarily with aligning the different waveguides to obtain maximum optical coupling at room temperature. Unfortunately, a change in ambient temperature causes a change in the properties of the waveguides, e.g. index of refraction, and therefore a shift in the center wavelength of signals transmitted therein.
A conventional slab-launched array waveguide grating (AWG), illustrated in FIG. 1, is integrated into a substrate 1, and includes a first slab waveguide 2 and a second slab waveguide 3, which are optically connected by an array of discrete waveguides 4. A plurality of discrete input/output waveguides 6 enable signals to be launched into or retrieved from the second slab waveguide 3. As is well known in the art, each of the waveguides 4 has a different length designed so that when a wavelength division multiplexed (WDM) signal is launched into either of the slab waveguides 2 and 3, the signal gets split into sub-beams, which travel along the different discrete waveguides 4 and interfere in the other slab waveguide, thereby creating sub-beams with discrete wavelengths. The discrete wavelengths can be output via the discrete input/output waveguides 6. Of course, this process is completely reciprocal and reversible, i.e. discrete wavelengths launched into either slab waveguide get combined into a single WDM signal for output the other slab waveguide.
As is disclosed in Japanese Patent Publication No. 04116607 published Apr. 17, 1992 in the name of Koga Masabumi et al, Japanese Patent Publication No. 06138335 published May 20, 1994 in the name of Takahashi Hiroshi, and several years later in U.S. Pat. No. 5,732,171 issued Mar. 24, 1998 in the name of Herbert Michel et al, it is, advantageous to mount at least one of the slab waveguides at the edge of the substrate to provide immediate access thereto for input or outputting signals. As illustrated in FIG. 1, it is convenient to mount the first slab waveguide 2 at the edge of the substrate 1 for receiving an input WDM signal directly from a fiber waveguide 7, the end of which is encased in a ferrule 8. Typically the ferrule 8 would be fixed directly to the edge of the substrate 1 in the position providing the highest possible optical coupling therebetween at an optimum set temperature. Without active temperature stabilization, changes in temperature encountered in use cause changes in the properties of the waveguides, e.g. the index of refraction, which results in shifts in the center wavelengths of the channels in the output signal. With reference to FIG. 2, various attempts have been made to compensate for optical coupling losses caused by changes in temperature by mounting one of the waveguides 7 on an expansion arm 9 that has a coefficient of thermal expansion (CTE) different than the support 10 fixed to the other waveguide 2. With this arrangement, the shift in the center wavelength caused by the change in temperature is partially or fully compensated for by a relative positional shift of the waveguides resulting from the expansion or contraction of the expansion arm. Examples of these devices are disclosed in Japanese Patent Publication 62211979 published Sep. 17, 1987 in the name of Hanamitsu Kiyoshi, Japanese Patent Publication No. 62237773 published Oct. 17, 1987 in the name of Nomura Hidenori, and World Patent Application WO 98/13718 published Apr. 2, 1998 in the name of Albrecht et al. Because of their cantilevered construction, the outer free ends of these devices are completely unrestrained in any direction. Accordingly, they can be relatively unstable, especially over time.
Improvements on the basic Albrecht et al design are disclosed in World Patent Applications Nos. WO 01/07948, WO 01/07949, and WO 01/07955 all published Feb. 1, 2001 in the name of Schweiker et al. Unfortunately, these applications only disclose spring elements extending in the expansion direction of the expansion arm, and fail to provide the means to restrict the end of the expansion arm from vibrating perpendicularly to the expansion direction.
An object of the present invention is to overcome the shortcomings of the prior art by providing a thermally compensated optical coupler, which is much more stable than the prior art over repeated usage.